When more than one write buffer (iomatrix) was getting used to compose a response, only the first buffer was getting sent.
Instead, try to call writev() again immediately if another iomatrix is pending (until writev() returns EAGAIN or some other error).
This defect has likely been present for a long time, since at least 0cc3309 (2010 Jun 22) when Feersum changed from write() to writev().